As semiconductor technology advances, device manufacturers work to reduce device sizes while they increase the density of devices on wafers. To do so, new methods and materials for device fabrication are necessary.
The move to replace standard aluminum interconnects with copper is one solution currently being implemented. Because copper has a lower resistivity than aluminum, an interconnect fabricated using copper has a lower resistance than an interconnect of the same dimensions fabricated using aluminum, which results in an increase in overall circuit speed.
However, using copper as an interconnect material poses some problems not encountered with aluminum. An important difference is that, unlike aluminum, copper does not form a self-passivating surface oxide. Therefore, as a wafer goes through different process environments and is subjected to oxidizing and reducing environments, a copper oxide layer may grow and/or shrink on the copper surface. As a result, copper films on the wafer may have an overlayer of copper oxide subsequent to processing, whose uncontrolled thickness depends on the wafer""s specific process history.
An oxide overlayer can cause a number of problems due to its chemical and electrical properties. For example, it may cause poor adhesion between the copper and subsequent layers. Additionally, when a subsequent metal layer is deposited to form an ohmic contact with the copper, an intervening oxide layer will increase the electrical resistance of the contact due to its highly insulating nature.
Due to the potential problems posed by the presence of an oxide layer on copper, an inexpensive and accurate method for determining the presence and/or thickness of a copper oxide layer is required to ensure device reliability. In addition, because there are two primary oxidation states of copper (Cu2O, cuprous oxide, and CuO, cupric oxide), the method should ideally be capable of measuring the thickness of either oxidation state. That is, the method should be applicable to films of CuxO where x=1 or 2.
There are many ways to measure the thickness of thin metal oxide overlayers with the precision and accuracy required for semiconductor processing applications, including surface analytical methods based on electron, photon, or ion spectroscopy (e.g. Auger Electron Spectroscopy (AES), X-Ray Photoemission Spectroscopy (XPS), Rutherford Backscattering Spectroscopy (RBS) and Secondary Ion Mass Spectrometry (SIMS)) and optical methods (e.g. spectroscopic reflectivity, ellipsometry, and glancing angle X-Ray Reflection (XRR)).
Optical methods are widely used for film metrology since they are non-destructive and practical for in-situ measurement, but these methods have not been developed for the commercially important case of ultrathin films ( less than 20 nm) of CuxO on Cu encountered in advanced integrated circuit (IC) processing. For example, many of these optical methods involve directing the light beam against the film at a shallow angle (i.e., almost parallel to the surface of the film). This technique has been found not to be useful for thin CuxO films, because these films are so transparent that substantial reflection takes place from the surface of the underlying substrate as well as the surface of the film. The difference between the light reflected by the surface of the substrate and the light reflected by the surface of the film cannot be xe2x80x9cseenxe2x80x9d by the detector.
Both XRR and ellipsometry can be used for copper oxide metrology but require expensive and complex apparatus. Additionally, optical reflectivity methods commonly used in IC processing are spectroscopic in that they probe the sample with a broad spectrum of wavelengths (for example, 200-800 nm) and then determine oxide film thickness by fitting a theoretical model of reflectivity to the measured spectrum. These methods require a costly spectrometer as well as substantial data analysis to perform the required curve fitting by which film thickness is ultimately determined.
We have recognized that the peculiar optical properties of copper oxide films allow for a considerable simplification in their thickness measurement. The current invention builds on this insight to provide an effective method and hardware for rapidly and cost-effectively determining the thickness of CuxO on Cu films with extremely high sensitivity without the need for sophisticated spectrometers or curve fitting algorithms.
In one embodiment, the current invention includes an apparatus for performing reflectometry using a specific wavelength or a small number of specific wavelengths to detect the presence of an oxide overlayer on copper and to measure its thickness. The invention also includes a method for obtaining reflectivity data and analyzing this data to determine film thickness.
The current invention provides rapid non-destructive measurement of CuxO-on-Cu thickness, with single atomic layer (about 5 xc3x85) sensitivity for ultrathin films with thickness about 200 xc3x85 (i.e. 20 nm) or less, but the method is not limited to such films. It is optimized for the commercially important case of CuxO-on-Cu, although it may be used with other materials that have an appropriate optical extinction coefficient, k, at one or more wavelengths. Although visible light is preferred for the case of CuxO-on-Cu, wavelengths outside of the visible spectrum may be practical for other materials. For example, infrared or ultraviolet light may provide acceptable thickness measurements for some types of films. As will be described later, we have found that the wavelength region from 350-480 nm is well suited for determination of CuxO-on-Cu thickness, with the region near 440nm providing an excellent choice due to the slope of its reflectivity versus thickness curve and the availability of a high-intensity light source.
The method works when the copper oxide is on the surface. The method can also be applied in the case where the oxidized Cu surface is partially reduced, such that the copper oxide is physically below the reduced copper surface. In this case, the Cu overlayer must be sufficiently thin ( less than 50 nm) to allow the incident visible light to reach and reflect from the subsurface layer of CuxO.
The method can be-performed using a suitable emission line source and band pass filter so that simple photodiode detectors may be used instead of a complex and costly spectrometer (although a spectrometer may be used to detect the reflected light if desired). Therefore, the invention can provide in-situ or vacuum integrated metrology with simple, low-cost hardware. Finally, the method does not require detailed curve fitting, and thus the necessary copper oxide thickness data can be acquired rapidly and without dependence on complex curve fitting algorithms which do not always converge on a unique thickness solution.
This invention can be more fully understood in light of the following detailed description taken together with the accompanying figures.